1. Field of the Invention
This invention relates to an apparatus used in semiconductor fabrication, and more particularly to an aligner detector, which uses an electrooptic modulator to select light with a specified light path.
2. Description of Related Art
A planarization technology in semiconductor fabrication is widely used to planarize an uneven surface of a wafer, resulting from various sub-structures formed on the wafer. After a planarization process, the wafer can have an even surface, which is very helpful to obtain precise fabrications for the subsequent fabrication processes, such as interconnect fabrication or pattern transfer. A chemical mechanical polishing (CMP) process is widely used in semiconductor fabrication to globally planarize the wafer surface. The CMP process may cause a zero mark on the wafer to be asymmetric. This results in an alignment error for each process. In order to obtain the alignment status, an aligner detector is used to analyze a diffraction light, which is a laser light beam diffracted by the zero mark.
FIG. 1 is schematic drawing of a conventional aligner detector. In FIG. 1, when a laser beam hits on a zero mark 10 at a desired location, the laser beam is diffracted into various individual diffraction light rays along different directions. The diffraction light rays pass through a wavy light refractor 12 and are refracted to a refractor set 20. The diffraction light rays travel in parallel after passing the wavy light refractor 12. According to the wavy structure, the diffraction light rays are assigned by an order number. The diffraction light rays 0, 1, and 7 are , for example, assigned as shown in FIG. 1. The higher order number means the farther diffracted. The refractor set 20 includes several wedge patterns with their fixed refraction angles, such as a wedge patter 20a and several wedge patterns 20b. The locations of the wedge patterns 10a, 20b are necessary to be properly matched to the diffraction light rays. For example, the wedge pattern 20a has a zero refraction and allows the diffraction light rays 0, 1 to pass. Each of the wedge patterns 20b has a certain refraction angle and is located at a position to receive a desired diffraction light ray, such as the diffraction light ray 7. The diffraction light rays again pass a wavy light refractor 14 and reach to a light detecting apparatus 30. The light detecting apparatus 30 includes a detector 30a, and several detectors 30b to receive the diffraction light rays. The detector 30a receives the diffraction light rays 0, 1, which are not refracted. The detectors 30b receive the other diffraction light rays, such as the diffraction light ray 7. After a proper arrangement the diffraction light rays with different light paths can reach to the detectors 30a, 30b.
For the conventional aligner detector described above, in order to detect the diffraction light rays of different orders, the light detector 30 has to include several sub-detectors 30a, 30b to detect some specified diffraction light rays. Since each detector has its different detector response, the measured signal of each detector is different. This may cause an alignment error. Moreover, each detector needs a calibration. The convention aligner detector including several detectors needs several calibrations. It is difficult to obtain a proper calibration.
In summary of above descriptions, the conventional aligner detector includes several detectors 30a, 30b. Since detector responses are different, the alignment error may occur due to an error analysis of detected signals of the detectors. It is difficult to properly calibrate the detectors. The fabrication cost is thereby increased.